High-density polyethylene which is in use in wide applications such as films, pipes, bottles and the like, has been conventionally prepared by using a Ziegler-Natta catalyst or a chromium catalyst. However, because of the nature of such catalysts, there has been limitation on the control over the molecular weight distribution or composition distribution of the polymer.
In recent years, several methods have been disclosed for preparation of an ethylene polymer having excellent moldability and mechanical strength, including an ethylene homopolymer or an ethylene.α-olefin copolymer of relatively small molecular weights and an ethylene homopolymer or an ethylene.α-olefin copolymer of relatively large molecular weights, according to a continuous polymerization technique, using a single-site catalyst which facilitates the control of the composition distribution, or a catalyst having such the single-site catalyst supported on a carrier.
In the publication of JP-A No. 11-106432, disclosed is a composition prepared by melt-blending a low molecular weight polyethylene with a high molecular weight ethylene.α-olefin copolymer, which are obtained by polymerization in the presence of a supported, geometric constraint type single-site catalyst (CGC/Borate-based catalyst). However, since the molecular weight distribution of the composition is not broad, fluidity of the composition may become poor. In addition, although the claims of the above-mentioned patent application do not disclose a preferred range of the carbon number of α-olefin that is to be copolymerized with ethylene, in the case of the carbon number being less than 6, it is expected that sufficient mechanical strength would not be exhibited. Further, because the molecular weight distribution (Mw/Mn) of the single-stage polymerization product is broad, it is also expected that the product's mechanical properties such as impact strength and the like would be insufficient, as compared with the single-stage product of narrower molecular weight distribution. Moreover, the anticipation that a broad composition distribution of the single-stage polymerization product would result in deterioration of the above-mentioned strength is obvious from the cross fractionation chromatography (CFC) data described in “Functional Materials,” published by CMC, Inc., March 2001, p. 50, and the cross fractionation chromatography (CFC) data described in FIG. 2 in the publication of JP-A No. 11-106432.
In the publication of WO 01/25328, disclosed is an ethylene polymer which is obtained by solution polymerization in the presence of a catalyst system comprising CpTiNP(tBu)3Cl2 and borate or alumoxane. This ethylene polymer has a weak crystalline structure due to the presence of a branch group in the low molecular weight component, and thus the polymer is expected to have poor mechanical strength. Also, since the molecular weight of the low molecular weight component is relatively large, it is expected that the polymer has low fluidity. Moreover, although the claims of the above-mentioned patent application do not disclose the preferred range of the carbon number of α-olefin that is to be copolymerized with ethylene, it is believed that when the carbon number is less than 6, sufficient mechanical strength would not be exhibited.
In the publication of EP 1201711 A1, disclosed is an ethylene polymer which is obtained by slurry polymerization in the presence of a catalyst system comprising ethylene.bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride with methylalumoxane supported on silica. Among such ethylene polymers, a single-stage polymerization product has a wide molecular weight distribution (Mw/Mn), and thus it is expected that the impact strength and the like would be insufficient as compared with a single-stage product of narrower molecular weight distribution. Further, it is inferred that a broad molecular weight distribution means heterogeneity of the active species, and consequently there is a concern that the composition distribution broadens, thereby resulting in deterioration of fatigue strength. Moreover, in some Examples of the above-mentioned patent application, a single-stage polymerization product of small molecular weight and a single-stage polymerization product of large molecular weight are melt-kneaded. In this kneading method, crystalline structures that are continuous over more than 10 μm are often produced, and thus it is expected that sufficient strength would not be exhibited.
In the publication of JP-A No. 2002-53615, disclosed is an ethylene polymer which is obtained by slurry polymerization using a catalyst system comprising methylalumoxane and a zirconium compound having a specific salicylaldimine ligand supported on silica. Although the claims of the patent application do not disclose the preferred range of the carbon number of α-olefin that is to be copolymerized with ethylene, in regard to the ethylene polymer obtained from 1-butene (number of carbon atoms=4) which is used as the α-olefin in Examples of the patent application, the carbon number is small, and it is envisaged that sufficient mechanical strength would not exhibited.
In general, an ethylene polymer shows a multimodal molecular weight distribution. When the intermodal molecular weight differences are large, mixing with melt-kneading is difficult, and thus multistage polymerization is typically employed. Multistage polymerization is in general often carried out in a continuous manner. In the case of such multistage polymerization, a distribution is usually created in the ratio between the residence time in a polymerization vessel which is under a polymerizing environment that would produce a low molecular weight product, and the residence time in a polymerization vessel which is under a polymerizing environment that would produce a high molecular weight product. In particular, in the case of a polymerization method in which the polymer is produced in a particulate form, such as the gas-phase method or slurry method, there may exist differences in the molecular weight among different particles. Such difference in molecular weight has been recognized even in the cases of using the Ziegler catalysts as described in the publication of Japanese Patent No. 821037 or the like. However, the catalyst is multi-sited, whereas the molecular weight distribution is broad. Accordingly, polymer particles are well mixed with each other even upon conventional pelletization by melt-kneading. On the other hand, in the case of using a singe-site catalyst, since the molecular weight distribution is narrow, the polymer particles are often not mixed sufficiently with each other during conventional pelletization by melt-kneading. Thus, in some cases, the history of polymer as having been in a particulate form was reflected in the mixture, and this caused disorder in the fluidity to adversely affect the appearance or sufficient exhibition of mechanical strength. Also, such ethylene polymer showed a tendency that the coefficient of smoothness which is determined from the surface roughness of extruded strands increased.
The ethylene (co)polymer prepared using a Ziegler catalyst as described in Japanese Patent No. 821037 or the like has methyl branch groups in the molecular chain as a result of side production of a methyl branch group during the polymerization reaction. It was found that this methyl branch group was embedded in the crystal, thus weakening the crystal (see, for example, Polymer Vol. 31, p. 1999 (1990)), and this caused deterioration in mechanical strength of the ethylene (co)polymer. Further, in regard to the copolymer of ethylene and α-olefin, when the copolymer contained almost no α-olefin, a tough and brittle component was produced; on the other hand, when an excessive proportion of α-olefin was used in copolymerization, a soft component with weak crystalline structure was produced, and thus it may cause tackiness. Moreover, since the molecular weight distribution was broad, there were problems such as the phenomenon of a low molecular weight product adhering to the surfaces of molded products as a powdery substance, and so on.
The ethylene polymer that is obtained by polymerization using a metallocene catalyst as described in the publication of JP-A No. 9-183816 or the like has methyl branch groups in the molecular chain, as a result of side production of a methyl branch group during the polymerization reaction. This methyl branch group is embedded in the crystals, thereby weakening the crystalline structure. This has been a cause for the lowering of mechanical strength. Also, an ethylene polymer with extremely large molecular weight has not been disclosed heretofore.
An ethylene polymer that is obtained by polymerization in the presence of a chromium-based catalyst exhibits low molecular extension because of the presence of a long-chained branch group, and thus has poor mechanical strength. Further, as a result of side production of a methyl branch group during the polymerization reaction, there exist methyl branch groups in the molecular chain. These methyl branch groups are embedded in the crystals and weaken the crystalline structure. This has been a cause for the lowering of mechanical strength. Further, in regard to the copolymer of ethylene and an α-olefin, when the copolymer contained almost no α-olefin, a tough and brittle product was produced; on the other hand, when α-olefin was copolymerized in an excessive proportion, tackiness was caused or a soft component with weak crystalline structure was produced.
The ethylene polymer that is obtained by polymerization in the presence of a constrained geometry catalyst (CGC) as described in the publication of WO 93/08221 or the like has methyl branch groups in the molecular chain, as a result of side production of a methyl branch group during the polymerization reaction. These methyl branch groups are embedded in the crystals and weaken the crystalline structure. This has been a cause for the lowering of mechanical strength. Further, the molecular extension was low because of the presence of long-chained branch groups, and thus the mechanical strength was insufficient.
An ethylene polymer that is obtained by high pressure radical polymerization has methyl branch groups or long-chained branch groups in the molecular chain, as a result of the side production of methyl branch groups or long-chained branch groups during polymerization. These methyl branch groups are embedded in the crystals, thereby weakening the crystalline strength. This has been a cause for the lowering of mechanical strength. Further, the presence of long-chained branch groups resulted in low molecular extension as well as a broad molecular weight distribution, and thus the mechanical strength was poor.
In regard to the ethylene polymer that is obtained by cold polymerization using a catalyst containing Ta- or Nb-complexes as described in the publication of JP-A No. 6-233723, since the ratio (Mw/Mn) of the weight average molecular weight (Mw) and the number average molecular weight (Mn) as measured by GPC was small, the moldability might be insufficient.